You get into the Blackrock castle, defeat a few guards and enemies spamming dynamite, Then. You meet him on his throne.

Cruel King

So, this is who I've heard so much about. You. Breaking in... dismantling my guards... Is this what you wanted? You think you're the hero of this story... But deep down, you're as filthy as everyone else. I've heard of the future. I've seen what's to come. I've been... 'tainted'... by a tormenting voice. With the knowledge that Blackrock Kingdom will cease to exist in your future. How do you think I feel? Withholding that knowledge from my people? That their happiness... their home... will be gone in only a few years. Do I give in to the prophesied future I've been told will unfold?

You say Yes.

Cruel King

... ..... No. I won't let that happen. With the Ice Dagger... I'll rewrite our history... No matter what it takes.

You and Cruel King get into battle as Cruel King gets off his throne, and pull out the Ice Dagger.